This invention relates to a shielding tape for telecommunication cables and the like
In the manufacture and application of cables for the transmission of high frequency signals, several parameters are highly important. Firstly, the conductor(s) transmitting the signal must be shielded to prevent signal loss by radiation, and also to prevent unwanted external radio frequency and electromagnetic interference from affecting the signals. It is also desirable that adjacent pairs of shielded conductors have such shields electrically insulated from each other to prevent "cross talk" between such adjoining pairs. Furthermore, such shielding must have an acceptable degree of flexibility to accommodate installation and working conditions, must not be excessively heavy or bulky, and must be economical to manufacture and incorporate into the cable. It is also desirable that the resistance of the shield be low enough to dissipate electrical currents impinging thereon, thus limiting the minimum thickness of the conducting layer which can be employed.
It is additionally desirable in certain applications that the shield exhibit circumferential continuity throughout its length, to eliminate the so-called "slot effect" caused by the insulating plastic layer bonded to the conducting layer at the overlap of the tape not allowing continuous circumferential contact of the metallic conductor layer at the overlap.
Prior art has employed a number of practices in attempting to meet the above conditions. The construction mainly used to attempt to meet these requirements is that known as Z-fold in which the above slit laminate has one edge folded back to expose the foil layer outermost and the other edge folded back in the opposite direction to expose the insulating layer outermost. One example of this construction is shown in U.S. Pat. No. 4,621,777 of the present inventor.
This product has been received favourably in the trade, but has a number of disadvantages. Firstly, the folding of the tape involves folding the laminate in its entirety including particularly the plastic insulating layer. This can exhibit springback resulting in an improperly shielded cable which would therefore have to be reworked, retaped or scrapped. In addition the extra thickness of plastic film adds unnecessary bulk and material to the tape.
The formation of tape and its winding into long length packages under current technology also necessarily involves splices in the tape along its length. These splices arise in individual layers before lamination, and in the tape after lamination and present a significant problem at the folding process. It is essential that the folding process is entirely consistent without any faults since an improper fold will cause an improperly shielded cable. Faults in the folding process are particularly prone to develop at the splices and accordingly must be closely monitored.
In addition, forming of the folded tape into a cylindrical package has raised some problems in that the folded edge of the tape which is wrapped on the inward side of the tape is held firmly, but the folded edge of the tape which is turned outwardly is inclined to unfold thus losing the essential consistency of wrapping in the finished cable product.
It will be appreciated that the total cost of manufacture of the finished cable product is heavily dependent upon the scrap or reject rate and hence any improvement in the consistency of the wrapping process can lead to significant reduction in cost.
Examples of tape construction are shown in U.S. Pat. Nos. 4,327,246 (Kincaid), 3,325,589 (Mildner), 3,474,186 (Hale) and 4,596,897 (Gruher). It is believed however that the disclosure of tapes in these patents is merely of a theoretical or simplified nature and the composite tapes apparently disclosed in these patents have been manufactured by laminating foil and plastic laminate to a plastic layer.